The present invention relates to an image-forming lens that forms an image of a photographic subject in order to image a still image or a moving image, and to an imaging apparatus, and an information device such as a portable information terminal device and the like having an imaging function using the image-forming lens. The image-forming lens is capable of being used in a silver-halide imaging apparatus using a silver-halide film, and in particular, the image-forming lens is suitable for an imaging apparatus using an electronic imaging device such as a digital camera, a digital video imaging apparatus, and the like.
As the market of so-called digital cameras has been getting larger and larger, requests regarding digital cameras from users are varied.
Among such digital cameras, a category of a small imaging apparatus with high-picture quality in which a relatively large image sensor, a diagonal length of which is about 20 mm to 45 mm, is used and a high-performance single focus lens is mounted is attracting attention from users.
A request from users in this category is heavily weighted toward not only high performance but also excellent portability, that is, miniaturization.
Here, in terms of high performance, at least, in addition to having a resolution capable of corresponding to an image sensor having about 10 to 20 million pixels (high resolution), less coma flare, high contrast and no point image distortion in a peripheral part of a field angle at an open aperture, less chromatic aberration and no occurrence of unnecessary coloring in a part having a large brightness difference, less distortion and capability of drawing a straight line as a straight line, and so on are at least needed.
Additionally, in terms of a large diameter, due to the necessity of making a difference from a general compact imaging apparatus having a zoom lens, at least a smallest f-number which is less than F2.8 where an aperture is open at a maximum is needed.
Moreover, in terms of miniaturization, in a high-resolution compact imaging apparatus, since a relatively large image sensor is used for a size of a body of the imaging apparatus, a real focal length is longer than that of a compact imaging apparatus having a small image sensor. Therefore, in order to achieve miniaturization having the high resolution image sensor, it is necessary to shorten an entire length of the image-forming lens in the imaging apparatus.
Furthermore, in terms of the field angle of a shooting lens, many users demand a rather wide angle, and it is preferable that a half-field angle of an image-forming lens be about 38 degrees.
The half-field angle of 38 degrees is equivalent to a focal length of 28 mm for a 35 mm silver-halide imaging apparatus using a traditional 35 mm silver-halide film (so-called a Leica format silver-halide film).
As an image-forming lens for a digital camera, many types are considered; however, as a typical constitution of a wide-angle single focus lens, there is a so-called retrofocus type image-forming lens having a lens group with a negative refractive power on an object side and a lens group with a positive refractive power on an image side.
An area sensor used as an image sensor has a characteristic such that each pixel is provided with a color filter or a micro lens. And there is a demand that an exit pupil position be distant from an image plane and a marginal light flux be incident onto the sensor at an angle close to perpendicularity. Those are the reasons for employing the retrofocus type image-forming lens.
However, in the retrofocus type image-forming lens, asymmetry of its refractive power arrangement is large, and corrections of comatic aberration, distortion, transverse chromatic aberration and the like tend to be incomplete.
In addition, in the retrofocus type image-forming lens, originally, the purpose is to ensure a back focus to use a wide-angle lens as a detachable lens for a single-lens reflex imaging apparatus. Therefore, as is clear from the above, the entire length of the lens (a distance from a most object side to a most image side of a lens system) tends to be longer.
Japanese patent application publication numbers 2010-39088, H09-96759, and the like disclose that such a retrofocus-type image-forming lens, in which the smallest f-number is less than 2.8, the half-field angle is around 38 degrees, and various aberrations can be favorably corrected.
The image-forming lens disclosed in Japanese patent application publication number 2010-39088 has the smallest f-number of about 1.9 and is bright. However, the entire length of the lens is more than nine times larger than a maximum image height, and is not sufficient for miniaturization.
Additionally, the image-forming lens disclosed in Japanese patent application publication number H09-96759 has a half-field angle of 41.5 degrees and is wide-angle. However, the entire length of the lens is more than six times larger than a maximum image height, and is not sufficient for miniaturization, either.